Fluidic laser calorimeter

ABSTRACT

A laser calorimeter which operates by absorbing the energy of a laser beam directed through a lens at the front of the calorimeter. The calorimeter contains a absorbing medium with high absorbance characteristics at the laser wavelength in question. The absorbed laser energy is converted to heat. The device has a means for measuring the changes in temperature of the absorbing medium as well as a means for measuring the changes in pressure inside the calorimeter chamber. The absorbing medium is typically a liquid which also demonstrates preferable heat transfer characteristics. This in turn affords greater precision and a faster response time of the calorimeter when measuring the laser energy. A typical medium for such a device is a dimethylformamide solution which has extremely good absorbance characteristics for an optical path length of 0.015 inches at a laser wavelength of 1.064 microns.

BACKGROUND OF THE INVENTION

The present invention relates to calorimeters and more particularly tothe means for accurately measuring energy of laser beams.

There exists a need for a simple and relatively low cost method ofaccurately measuring the energy levels of numerous scientific andindustrial lasers. Calorimeters are a standard device used in mosthigh-energy laser applications and their development. However, acalorimeter capable of measuring the laser energy with great precisionand with a fast response time is often complex and tends to beexpensive.

The prior art in this area of energy measurement for lasers typicallyuse radiation detectors to measure the power of the laser beams. Suchenergy measuring devices have limited power ranges and limited accuracydue to the presence of detector noise. The use of a thermopile in laserpower meters is also known as a common method to measure laser energy.These meters use a block of metal which is heated with the laser beam.The temperature rise in the metal block is used to determine the laserpower. Accurate models of such devices are available but tend to berelatively expensive. The less expensive models have poorer accuracy.

SUMMARY OF THE INVENTION

It is the object of this invention to provide a device for accuratelymeasuring the energy of a laser beam.

It is a further object of this invention to provide a device forconverting the energy of a laser beam to thermal energy or heat.

Yet another object of this invention is to provide a device forabsorbing the energy of a laser beam of specified wavelengths.

It is still a further object of this invention to provide a device formeasuring the energy of individual pulses of a pulsed laser source.

It is still a further object of this invention to provide a device forconverting the energy of a pulsed laser source to an increase ininternal pressure and thermal energy.

It is still a further object of this invention to provide a device formeasuring the repetition rate of a pulsed laser source.

It is still a further object of this invention to provide a device forabsorbing the energy of a pulsed laser source at a specified wavelength.

It is still a further object of this invention to provide a device fordetermining the absorbance characteristics of fluidic or gaseous mediumsat specified wavelengths.

A key feature of this invention is a simple method of modifying oradjusting the device to increase or decrease the measurement sensitivitywith respect to the various wavelengths of the laser beams to bemeasured.

Another key feature of this invention is the utilization of a fluidmedium, such as a liquid or a gas, for absorbing the energy of a laserbeam of specified wavelengths.

Yet another feature of this invention is the capability of measuring thetemperature changes and pressure changes in a confined chamber which hasbeen subjected to a directed laser energy source. There exist manyalternative configurations to the present invention which employ varyingtechniques of measuring the temperature and pressure changes within thechamber. Such flexibility permits this device to be tailored to aspecific application, integrated within a more complex apparatus or tobe used in stand-alone applications.

The present invention is a laser calorimeter. It provides an accuratemeans to measure the energy of a continuous or pulsed laser beam at arelatively low cost. A laser beam is directed at the calorimeter andpasses through a lens into the calorimeter chamber. The calorimeterchamber is filled with an absorbent fluid medium in a liquid or gaseousstate. Once inside the chamber the laser energy is absorbed by theabsorbent medium. The absorbed energy is converted to thermal energy orheat thereby causing the temperature of the medium to rise. The internalchamber pressure will also rise. The present invention utilizes variousmeans to measure the increase in temperature and pressure. The resultingincreases in temperature and pressure can then be used to calculate theenergy absorbed by the medium.

The laser energy of individual pulses for a pulsed laser can also beabsorbed, converted to alternate energy forms, and ultimately calculatedor measured. A pressure transducer or other similar device presents thebest alternative within the present embodiment for converting laserenergy to an alternate energy form and determining the number ofindividual pulses over a specified time frame. This value together withthe measured value of the total laser energy absorbed, as described inthe preceding paragraph, can be used to calculate the energy ofindividual pulses.

The present invention allows the use of different absorbent mediums,such as liquid dyes, within the calorimeter chamber which alters theabsorbing characteristics of the calorimeter. Depending on theapplication and design, one can achieve near total absorption, orpartial absorption if so desired. Further, it is possible to determinethe absorption characteristics of a known or unknown medium at specificwavelengths, if the energy output and wavelength of the source areknown. If only partial absorption is desired, or the absorptioncharacteristics are to be determined, the laser beam would pass throughthe medium and exit the calorimeter through another lens. In this mannerit is also possible to have the present invention act more as a filterby absorbing certain wavelengths of laser light and allowing otherwavelengths to pass through.

One important feature of the present invention is the ability to controlthe volume of the fluid dye solution or concentration of the fluid dyesolution or both. Such control allows for greater resolution, accuracy,and sensitivity of the calorimeter with various lasers and the length oftime exposed to the laser energy.

The present invention satisfies the aforementioned objectives andincorporates the preceding features in a manner that will be apparentfrom consideration of the drawings and the detailed description of theinvention which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view of the present invention in a single lensconfiguration.

FIG. 2 is similar to FIG. 1, showing the present invention in analternate configuration for measuring internal pressure as well astemperature.

FIG. 3 is a cross section view of the present invention in a dual lensconfiguration.

DETAILED DESCRIPTION

Referring to FIG. 1 the present invention (12) in a preferred embodimentis shown consisting of a housing (10) and a lens (20) attached to anopening (13) at a front end of the housing 10) thereby creating anenclosed chamber (40). The chamber (40) contains an absorbent solution(14) which is shown here as a liquid dye solution. The interior surface(11) of the housing (10) can be mirrored so as to cause the light energyto reflect back through the absorbent solution (14) prior to escapingand avoids any energy being absorbed by the housing (10). This featureimproves the overall accuracy and sensitivity of the device. By passingthe laser energy through the absorbent solution (14) multiple times, thevolume of the absorbent solution (14) needed to absorb nearly all theenergy can be reduced. A reduced volume of the absorbent solution (14)will cause greater temperature rises for a given amount of absorbedenergy.

Also shown in the chamber (40) and protruding through the housing (10)is a thermistor (30) which is used to measure the temperature of theabsorbent solution (14). The thermistor is connected to a automatedrecording device such as a computer (not shown) or other peculiar testequipment (not shown) which records the resultant values of thethermistor (30) and calculates the absorbent solution (14) temperature.The temperature measurements can then be used to determine the energyabsorbed by the absorbent solution (14). The use of a thermistor (30) inthe present invention is merely one method of measuring temperature.Various alternatives such as a thermocouple or other thermometer devicecan be used with the invention in lieu of a thermistor (30). Further,the absorbent solution (14) can be replaced with any liquid or gaseousmedium possessing the absorbent characteristics desired for the givenapplication or use.

As stated earlier, the present invention is a laser calorimeter (12). Alaser beam (not shown) of a specified wavelength is directed toward thedevice and passes through the front lens (20) and into the chamber (40).The chamber (40) contains a liquid dye solution which has specificabsorbance characteristics at the laser wavelength in question. In thepreferred embodiment a liquid dye solution containing dimethylformamidesolvent is contained within the chamber (40). Specifically, the liquiddye solution used in the prototype invention contained one milliliter ofdimethylformamide solvent per 55 milligrams of dye. This dye solutionhas a very high absorbance at the ND:Yag wavelength of 1.064 microns. Atthe concentration stated above approximately 1×10.sup.(-50) of the lightwould not be absorbed at an optical path length of 0.015 inches. It isclear however, that numerous dye solutions could be used as theabsorbent solution (14) in the present invention (12). Further, thecharacteristics, materials, and dimensions of the housing, chamber, andlens can be tailored to the specific application. The overridingconsideration when making such selections is the compatibility of thevarious components including the laser wavelength and power, propertiesof the absorbent, and length of time the present invention is exposed tothe laser beam. A preferred embodiment of the present invention uses athree-fourths inch diameter circular glass convex lens of predeterminedthickness, a small stainless steel housing approximately two inches bytwo inches by one-half inch and having an chamber volume of less thantwenty cubic centimeters. Alternate lens configurations include but arenot limited to a convexo-convex, convexo-concave, plano-convex,plano-concave, concavo-convex, or concavo-concave lens.

As a laser beam (not shown) enters the chamber (40) it is absorbed bythe absorbent solution (14). All the absorbed laser energy is convertedto heat. By measuring the temperature rise in given volume of absorbentsolution (14) the amount of heat generated and correspondingly, theamount of absorbed energy can be determined.

An alternate configuration of the present invention is show in FIG. 2.This device is similar to the device shown in FIG. 1 but has additionalfeatures. As can be seen, this device consists of a housing (10) and alens (20) attached to the opening (13) at a front end of the housing(20) thereby creating an enclosed chamber (40). The chamber (40)contains an absorbent solution (14) which is shown here as a fluid dyesolution similar to that described above. The interior surface (11) ofthe housing (10) can be mirrored as described above so as to cause thelight energy to reflect back through the absorbent solution (14) priorto escaping and avoids any energy being absorbed by the housing (10).Also shown in the chamber (40) and protruding through the housing (10)is a thermistor (30) which is used to measure the temperature of theabsorbent solution (14). The thermistor (30) is connected to a automatedrecording device such as a computer (not shown) or other peculiar testequipment (not shown) which records the resultant values of thethermistor (30) and calculates the temperature of the absorbent solution(14). The temperature measurements can then be used to determine theenergy absorbed by the absorbent solution (14).

An additional feature of this configuration is the pressure transducer(15) which measures the pressure changes in the calorimeter chamber(40). As shown in FIG. 2, the pressure transducer (15) is located on oneside of the housing (10). The pressure transducer (15) shown in thisconfiguration is a bellows structure (50), having an interior cavity(52). This bellows structure (50) converts the pressure variations orchanges into linear motion. A linear variable differential transformer(LVDT) (16) is attached to the end of the bellows structure (50) andaligned along the axis of linear motion (51).

The linear variable differential transformer (LVDT) (16) is comprised ofa transformer core 70), a transformer coil (60) of an electrical circuit(not shown) disposed slightly above the transformer core (70) and asecond transformer coil (80) of an electrical circuit (not shown)disposed slightly below the transformer core (70). The transformer core(70) is rigidly attached to the end of the bellows structure (50) suchthat the transformer core (70) moves simultaneously along the samemotion axis (51) as the bellows structure (50) when the bellowsstructure (50) is expanded and compressed due to pressure variationsinside the chamber (40). The transformer coils (60,80) are an integralpart of electrical circuits (not shown) whose output voltages aremeasured and recorded on a device such as a computer (not shown) orother peculiar test equipment (not shown) which records the resultantoutput values of the circuits, and calculates and monitors over time theinternal pressure of the chamber (40). The pressure measurements canthen be used to determine the number of distinct pulses of a pulsedlaser energy source and ultimately the laser energy per pulse.

The use of a bellows structure (50) in the present invention is merelyone method of measuring pressure variations. Various alternatives suchas a conventional diaphragm or other pressure sensing devices can beused with the invention in lieu of a bellows structure (50).

The manner of operation of the configuration shown in FIG. 2 is similarto that described above. A pulsed laser beam (not shown) of a specifiedwavelength is directed toward the device and passes through the frontlens (20) and into the chamber (40). The chamber (40) contains anabsorbent solution (14) which has specific absorbance characteristics atthe laser wavelength in question. As discussed above, numerous dyesolutions could be used as the absorbent solution (14) in the presentinvention.

As the pulsed laser beam (not shown) enters the chamber (40) it isabsorbed by the absorbent solution (14). By measuring the pressurevariations over time in the chamber (40) due to the fluid dye solutionabsorbing the laser energy, it is possible to determine the number ofpulses in a given time period. Each individual pulse will cause adiscrete rise in the internal pressure of the chamber (40). This allowsthe measurement of the laser repetition rate. The laser repetition ratealong with the measured power allows the determination of energy perpulse for a pulse laser source. The total laser energy of a pulsed lasersource can be measured as discussed above using a temperature sensorsuch as a thermistor (30) in addition to a pressure sensing device.

Yet another configuration is shown in FIG. 3. This device is alsosimilar to the devices shown in previous figures but has additionalfeatures. As can be seen, this device consists of a housing (10) and afirst lens (20) attached to the opening (13) at the front end of thehousing (10) and a second lens (90) attached to the opening 16) in theaft end of the housing (10) thereby creating an enclosed chamber (40).The chamber (40) contains an absorbent solution (14) which is shown hereas a fluid dye solution similar to that described above. Also shown inthe chamber (40) and protruding through the housing (10) is a thermistor(30) which is used to measure the temperature of the absorbent solution(14). The thermistor (30) is connected to a automated recording devicesuch as a computer (not shown) or other peculiar test equipment (notshown) which records the resultant values of the thermistor (30) andcalculates the temperature of the absorbent solution (14). Thetemperature measurements can then be used to determine the energyabsorbed by the absorbent solution (14).

The additional features of this configuration is the presence of asecond lens (90) which enable this device to filter laser light atspecified wavelengths by absorption as described above while permittingthe other wavelengths to pass through. Also, the presence of theremovable plug (21) allows the absorbent solution (14) to be replaced asthe design and application requirements dictate. Clearly, this plug (21)can also be employed in any of the configurations of the presentinvention as described above.

Furthermore, use of the dual lens configuration allows the determinationof absorbent characteristics of various absorbent solutions (14) atspecified wavelengths. Knowing the energy and wavelength of the directedlaser beam (not shown), and the optical path dimensions or distancebetween the first lens (20) and the second lens (90) of the device, theabsorbent characteristics can be calculated by measuring the energyabsorbed by the absorbent solution (14) within the housing chamber (40)using the thermistor (30) as discussed earlier in the operation of thebasic laser calorimeter.

What I now claim as the invention is:
 1. A laser calorimeter formeasuring laser energy comprising:a housing having a forward enddefining an opening and an interior chamber, said chamber having aninterior surface opposite said opening; a lens disposed in said openingof said housing and attached peripherally thereto whereby said laserenergy may enter said housing chamber; a means disposed in said chamberfor absorbing said laser energy in said housing chamber; and a meansextending sealably through said housing and into said chamber formeasuring changes in temperature in said means for absorbing said laserenergy.
 2. The laser calorimeter for measuring laser energy of claim 1further comprising a means extending sealably through said housing andinto said chamber for measuring changes in pressure in said housingchamber.
 3. The laser calorimeter of claim 2 wherein said interiorsurface opposite said lens comprises a mirror surface.
 4. The lasercalorimeter of claim 2 wherein said lens comprises a plano-convex lens.5. The laser calorimeter of claim 2 wherein said means for measuringchanges in temperature in said housing chamber comprises a thermistor.6. The laser calorimeter of claim 2 wherein said means for absorbingsaid laser energy comprises a liquid dye solution disposed in saidhousing chamber.
 7. The laser calorimeter of claim 6 wherein said liquiddye solution comprises a mixture having known specific absorbancecharacteristics of laser energy at specific wavelengths.
 8. The lasercalorimeter of claim 7 wherein said liquid dye solution comprises amixture containing insert dye solution and dimethylformamide.
 9. Thelaser calorimeter of claim 7 wherein said liquid dye solution comprisesa mixture containing 55 mg of said dye solution per milliliter ofdimethylformamide.
 10. The laser calorimeter of claim 2 wherein saidmeans for measuring changes in pressure comprises a pressure transducerattached to said housing and extending sealably into said chamber. 11.The laser calorimeter of claim 10 wherein said pressure transducercomprises:a bellows structure having an axis of linear motion andfurther having a cavity; a linear variable differential transformerdisposed on said bellows structure along said axis of linear motion; anda means for attaching said pressure transducer to said housing so thatsaid chamber extends sealably into said bellows cavity.
 12. The lasercalorimeter of claim 11 wherein said linear variable differentialtransformer comprises:a transformer core component having a first sideand a second side opposite said first side; a transformer coil elementof an electrical circuit disposed near said first side of saidtransformer core; a transformer coil element of an electrical circuitdisposed near said second side of said transformer core; and a means forattaching said transformer core to said extension bellows whereby saidtransformer core moves in the direction of the axis of linear motionwith pressure changes inside said chamber.
 13. A laser calorimeter formeasuring laser energy of a laser beam comprising:a housing having aforward end defining a first opening, an interior chamber, and an aftend defining a second opening opposite said first opening; a first lensdisposed in said first opening of said housing whereby said laser beammay enter said housing chamber; a means disposed in said chamber forabsorbing said laser energy of said laser beam; a means extendingsealably through said housing and into said chamber for measuringchanges in temperature in said chamber; and a second lens disposed insaid second opening of said housing whereby said laser beam may exitsaid housing chamber.
 14. The laser calorimeter of claim 13 wherein saidfirst lens and said second lens comprise a first plano-convex lens and asecond plano-convex lens respectively.
 15. The laser calorimeter ofclaim 13 wherein said means for measuring changes in temperature in saidhousing chamber comprises a thermistor.
 16. The laser calorimeter ofclaim 13 wherein said means for absorbing said laser energy of saidlaser beam comprises a liquid dye solution disposed in said housingchamber.
 17. A laser calorimeter for measuring laser energy comprising:ahousing having an interior chamber; an optical means for communicatingsaid laser energy with said chamber, said means extending sealablythrough said housing so that said laser energy may pass through saidoptical means; a means disposed in said chamber for absorbing said laserenergy in said housing chamber; and a means extending sealably throughsaid housing and into said chamber for measuring changes in temperaturein said means for absorbing said laser energy.